1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount for damping or isolating vibrations based on resonance of a fluid contained therein. More particularly, this invention is concerned with such a fluid-filled cylindrical elastic mount capable of exhibiting excellent vibration damping or isolating effect based on the resonance of the fluid flows, over a wide range of frequency of the input vibration.
2. Discussion of the Prior Art
There has been a growing requirement for improving the operating characteristic and function of a vibration damping or isolating device for flexibly connecting two members. In view of the difficulty in satisfying the requirement on a conventional elastic vibration damper or isolator which relies solely on the elastic nature of an elastic body to provide a vibration damping or isolating effect, an elastic mount or bushing filled with a fluid has been recently proposed.
An example of such a fluid-filled elastic mount is disclosed in laid-open Publication No. 56-164242 of unexamined Japanese patent application, wherein an elastic body made of a rubbery material is formed between an inner and an outer sleeve which are radially spaced apart from each other. The elastic body flexibly connects the inner and outer sleeves, so that two members of a vibration system that are fixed to the inner and outer sleeves are connected to each other by the elastic mount in a vibration damping or isolating manner. Between the inner and outer sleeves, there are formed a plurality of fluid chambers filled with a suitable non-compressible fluid. The fluid chambers are held in communication with each other through an orifice passage. Upon application of a vibrational load in a diametrical direction of the elastic mount, the fluid is forced to flow between the fluid chambers through the orifice passage. This type of fluid-filled cylindrical elastic mount relies on the resonance of the fluid flowing through the orifice passage, and exhibits an excellent vibration damping or isolating effect that cannot be obtained with the conventional elastic mount or damper which uses only an elastic body.
The fluid-filled cylindrical elastic mount of the type described above is capable of effectively damping or isolating input vibrations received in various diametrical directions, and can be readily adapted to prevent an excessive amount of relative displacement of the two members flexibly connected by the elastic mount. Further, this elastic mount can be made compact and small in size. For these advantages, the fluid-filled cylindrical elastic mount is suitably used as an engine mount, a differential mount, a member mount and a suspension bushing for motor vehicles. These elastic mounts used for the vehicles are usually subject to various vibrations having different frequencies, which occur depending upon the varying running conditions of the vehicles. Accordingly, the elastic mounts are required to exhibit different operating characteristics for effectively damping or isolating such different bands of the input vibrations, i.e., to deal with a relatively wide range of the input vibrations.
For instance, an engine mount for a motor vehicle is required to exhibit a low dynamic spring constant with respect to medium-frequency vibrations such as engine idling vibration having a frequency range of about 20-50 Hz, while the vehicle is stopped with the engine running in an idling condition. While the vehicle is cruising, the engine mount is also required to exhibit a high damping effect with respect to low-frequency vibrations such as engine shake and bounce having frequencies in the neighborhood of 10 Hz, and also exhibit a low dynamic spring constant with respect to high-frequency vibrations such as booming noises having frequencies in the neighborhood of 100 Hz.
However, the conventional fluid-filled cylindrical engine mount exhibits a sufficient damping or isolating effect based on the resonance of the fluid, with respect to only the vibrations whose frequencies fall within a relatively narrow range including the specific resonance frequency obtained by tuning the orifice passage. Hence, the known fluid-filled cylindrical engine mount is not satisfactory in terms of the above-indicated different requirements. If the resonance frequency of the orifice passage is tuned to provide a high damping effect with respect to the engine shake and bounce or other low-frequency vibrations having frequencies in the neighborhood of 10 Hz, the orifice passage operates as if it was substantially closed when the engine mount receives medium- to high-frequency vibrations whose frequencies are considerably higher than 10 Hz. In this case, therefore, the engine idling vibrations and booming noises cannot be effectively damped or isolated. If the resonance frequency of the orifice passage is tuned to exhibit a sufficiently low dynamic spring constant with respect to the idling vibrations or other medium-frequency vibrations of 20-50 Hz, then the engine mount is not sufficiently capable of damping the low-frequency vibrations such as the engine shake and bounce, and the dynamic spring constant exhibited by the engine mount is not sufficiently low with respect to the high-frequency vibrations such as the booming noises.